Hot gas in an aircraft arises, for example, in the course of operation of an auxiliary aggregate (in the following also called “Auxiliary Power Unit” or APU, respectively) which generates hot gas in the form of waste gas having a temperature of up to 730° C. A waste-gas system that cools and conducts away waste gas has to be mechanically connected to the structure of the aircraft and simultaneously be capable of equalising relative movements due to operation, thermal expansions, tolerances of manufacture and assembly.
In the case where use is made of ducts or channels for transporting gases, relative movements or production tolerances are ordinarily preferably compensated through the use of elastic rubber elements. By reason of the high temperatures arising in the case of pipes perfused by hot gas, the use of temperature-sensitive rubber—such as, for example, a rubber bellows—alone is no longer possible.
Furthermore, bellows made of metal are employed in addition which, although temperature-resistant, require certain minimal dimensions in order to be able to equalise predetermined relative movements. The minimal dimensions conflict with a requisite compact and effective structure in an aircraft, so that bellows made of metal are disadvantageous for the desired field of application.
In an aircraft, for instance, when conducting away APU waste gases, devices for positional equalisation are frequently used that can be compared with a piston/piston-ring combination. In the case of an APU arranged in the tail of an aircraft, such a technical solution is adequate for compensating the static and dynamic dimensional deviations between the APU, by way of system generating hot gas, and the waste-gas system. In this connection the APU and the waste-gas system are fastened to the same structural section. In the case where the APU and the waste-gas system are not fastened to the same structural section, both dynamic and static dimensional deviations that arise between the structural sections involved have to be taken into account. For instance, in the case of the military transporter of the AIRBUS A400M type the waste-gas system is fastened to the wing/fuselage fairing, and both the APU and an associated firewall are fastened to the fuselage. The dimensional deviations of the various structural sections therefore have to be taken into account in addition to the dimensional deviations between the system generating hot gas and the waste-gas system. Compensating the additive dimensional deviations between the APU and the waste-gas system and also, in addition, the dimensional deviations between the two structural sections by means of the proven piston/piston-ring combination could only be realised with an appropriately high piston-ring height. Associated with this, a correspondingly distinct increase in the restoring forces of the piston ring in the installed state would occur, resulting in increased friction. By virtue of the larger relative movements, in addition the forces due to the mass accelerations would increase. By reason of the thermal loading by the hot gas the structural members for conducting hot gas would be unable to support the high friction forces, as a result of which damage may arise.
DE 690 19 36012 relates to a flexible coupling device for a waste-gas or exhaust pipe manifold system of an internal-combustion engine, which serves to connect a first manifold section to a second manifold section. Formed on the first manifold section is a flange extending radially outwards, to which a first end of a flexible bellows is fastened by means of a first retaining ring. At its end facing towards the first manifold section the second manifold section likewise exhibits a flange projecting radially outwards, which forms a flat fastening surface. A second retaining ring is connected to the flange formed on the second manifold section and further secures a guide ring as well as a second end of the flexible bellows.
DE 20 2004 019 988 U1 discloses a connecting element for non-rigid connection of the end regions of two pipelines, which comprises a heat-protection element consisting of a mounting-pipe part and a protecting-pipe part for protection of a bellows element from hot gases flowing through the pipelines. Between end regions of the mounting-pipe part and the protecting-pipe part a flexible-spring arrangement may be provided.